The importance of oxygen binding to various types of heme units in biological molecules is well known especially in regard to O2 transport and the respiratory process. The interaction of manganese with O2 is postulated in such reactions as photosynthetic O2 evolution in chloroplasts and the scavanging of superoxide ion by superoxide dismutase. Being a trace metal in rather fragile systems creates experimental problems regarding the role and nature of manganese. A detailed investigation into the interaction of oxygen with relatively simple manganese coordination compounds is proposed. The research program envisioned is fivefold: (1) preparation and characterization of potential manganese-dioxygen precursors employing a range of polydentate ligands, (2) synthesize and study the oxygenation products of mangenese (II) chelates, (3) measure core orbital ionization energies of manganese and oxygen in oxygenation products and precursors via X-ray photoelectron spectroscopy, (4) measure valence ionization energies of manganese (II) complexes via mass spectrometry and (5) study the redox properties of manganese (II) compounds as a function of donor atom. Factors to be elucidated include the structural requirements for reversible O2 uptake, the nature of the manganese-oxygen linkage and the role of manganese in photosynthetic O2 evolution and superoxide dismutase.